Form parent class must be AbstractType

[flip111]

This section of the documentation could tell that the parent must always be AbstractType

[wouterj]

This is not true. However, there is a big difference between the Form hierarchy and the PHP inheritance. Using both does not work. We may should add that into details to a cookbook article.

[flip111]

If it's not true then that means that sometimes you can use PHP inheritance and sometimes not. Can we make a list of situations where it's good to use it and where not good to use it?

[stof]

Well, you could use PHP inheritance if you don't use the form type hierarchy, but I don't see any use case for that. You would loose some features without gaining any AFAIK.

[wouterj]

@stof but we can't say that we should force extending AbstractType. Take the button groups for example. Each type need to implement FormTypeInterface, that's the only requirement.

However, it extending a type (for instance ChoiceType) and setting choice as parent in the form hierarchy leads to unexpected behaviour (at least, if you don't understand what's going on internally).

So maybe we should add cookbook article (if we haven't already) about creating a form type. In it, we will just show how create form types without any inheritance stuff. However, in a last section of the article, we will explain the Form inheritance system.

[stof]

@wouterj the rule is that you should probably never extend an existing form type class. Then, you can either extend AbstractType or reimplement the whole interface yourselves (even though I don't think about any reason for this).

[wouterj]

I propose to create a new cookbook recipe in cookbook/form that talks about the Form inheritance model, as many are confused by it. This should also explicitely warn users to not extend form types using PHP.

[flip111]

👍

[linaori]

Seems like this information is still missing. Should consider if virtual inheritance still the way to go.

[stof]

well, using PHP inheritance allows you to reuse the logic in the form type. But it does not make the Form component aware of a hierarchy between these types, and so the theming layer will consider them as totally unrelated (and type extensions too). So yes, the type hierarchy is still relevant.